The invention relates generally to electrotherapy and, more particularly, to delivery of electrotherapy to patients having different physiological characteristics.
One significant challenge to manufacturers of medical equipment that provide electrotherapy to patients is patient variability. Different patients have different physiological characteristics, such as transthoracic impedance, that may affect the efficacy of treatment provided to them. A device that provides electrotherapy to one patient with success may not be successful when providing the same treatment to a different patient because of differences in the patient""s physiological make-up.
Ventricular fibrillation is one life-threatening medical condition that is treated by application of electrotherapy. Electrotherapy in the form of a defibrillation pulse must be strong enough to stop the heart""s unsynchronized electrical activity and give the heart a chance to reinitiate a synchronized rhythm. Resistance to the flow of electrical current through a patient""s thorax is known as transthoracic impedance, and is typically measured in ohms.
An external defibrillator may be used with different patients having a wide range of transthoracic impedance. Conventional defibrillators are often specified for and tested with 50 ohm loads (to represent a xe2x80x9cstandardxe2x80x9d or xe2x80x9creferencexe2x80x9d patient). However, the actual transthoracic impedance of patients can vary greatly in a range from 20 to 300 ohms, though most patients typically fall in a range of 25 to 180 ohms.
Defibrillation pulses that are effective (i.e., successful) in treating low impedance patients may not necessarily deliver effective treatment to high impedance patients, and vice versa. For instance, too little current in a defibrillation pulse may not be effective in defibrillating the patient""s heart. On the other hand, too much current in a defibrillation pulse can be inefficient, and more importantly, may damage a patient""s tissue. The risk of damage to a patient""s myocardium from a large defibrillation shock suggests that defibrillation should be attempted with the lowest current practicable, consistent with a desired probability of success for the defibrillation therapy.
Different approaches exist in the prior art for delivering defibrillation therapy to patients having different transthoracic impedance. One approach seeks to deliver a constant amount of current to each patient, regardless of the patient""s impedance. The defibrillation pulse generated by the defibrillator is adjusted in response to a measurement of the patient""s impedance so that the current delivered by the defibrillator to each patient remains constant. In practice, however, it is difficult to achieve delivery of a constant amount of current in an external defibrillator. See, e.g., U.S. Pat. No. 5,908,442 to Brewer, which describes delivery of a stepped truncated defibrillation waveform.
Another approach to delivering defibrillation therapy is to measure the patient""s impedance and adjust the defibrillation pulse to maximize the patient""s cardiac response to the defibrillation pulse. For instance, U.S. Pat. No. 5,908,442 further describes computation of a waveform duration that is intended to maximize the response of the patient""s cardiac cell membranes to the defibrillation pulse. The computation is derived from a defibrillator circuit and patient model, which includes a representation of the patient""s transthoracic impedance. However, when attempting to maximize the response of a patient""s cardiac cell, more voltage may be applied to the patient than is needed to successfully defibrillate the patient, potentially damaging the patient.
Moreover, delivering a defibrillation pulse intended to maximize a patient""s cardiac cell response results in different patients receiving different levels of defibrillation therapy that may or may not be effective in treating the patient. The present invention provides a solution for these problems and other shortcomings in the prior art.
The present invention provides a medical device and method for delivering electrotherapy to different patients that results in an equivalent probability of success in the different patients. A medical device constructed according to one exemplary embodiment of the invention includes electrodes adapted to be placed on a present patient, a measuring unit in communication with the electrodes for measuring a patient-dependent electrical parameter, such as impedance, of the patient, and an electrotherapy generator in communication with the electrodes for delivering electrotherapy to the patient. The medical device further includes a processing unit for controlling the electrotherapy delivery to the patient. The processing unit is configured to cause the electrotherapy generator to deliver electrotherapy that is adjusted for the patient based on the patient""s measured patient-dependent electrical parameter. In one implementation, a predetermined response of a reference patient to a nominal electrotherapy is used. The actual electrotherapy delivered to the present patient is controlled so that the electrotherapy has a probability of success for the present patient that is equivalent to the probability of success of the nominal electrotherapy for the reference patient. One implementation of the invention produces a model response in the present patient that is equivalent to the predetermined response of a reference patient model to the nominal electrotherapy.
In accordance with one aspect of the invention, a nominal electrotherapy is selected for delivery to a present patient. During a preprocessing stage, the response of a reference patient to the nominal electrotherapy is determined. The reference patient is characterized to have a patient-dependent electrical parameter (such as impedance) of a reference amount. A corresponding patient-dependent electrical parameter is measured in the present patient. The delivery of electrotherapy to the present patient is then controlled based on the patient""s measured patient-dependent electrical parameter and the determined response of the reference patient to the nominal electrotherapy. The actual electrotherapy delivered to the present patient has a probability of success that is equivalent to the probability of success of the nominal electrotherapy for the reference patient.
The delivery of electrotherapy to a present patient may be controlled in accordance with a translation factor determined during a preprocessing stage. The preprocessing stage in one embodiment of the invention includes determining the model response of a reference patient to a nominal electrotherapy. Also determined are the model responses of different patients to the nominal electrotherapy. The preprocessing includes comparing the model response of the reference patient with the model response of the different patients to the nominal electrotherapy, and determining an adjustment that compensates for relative differences between the model response of the reference patient and the model response of the different patients. The determined adjustment is used to modify the nominal electrotherapy to produce an actual electrotherapy for a present patient. The actual electrotherapy produces a model response in the patient that is equivalent to the model response of the reference patient to the nominal electrotherapy, with an equivalent probability of success. The patient-dependent electrical parameter used in characterizing the different patients may be an impedance of the patients, such as transthoracic impedance.
In another embodiment of the invention, the preprocessing stage includes evaluating the response of a number of different patients to electrotherapy. By empirical analysis, levels of electrotherapy are determined for different patients that result in a probability of success that is equivalent for all of the patients. While there may not be an explicit predetermined response of a reference patient model that is used to guide the determination of actual electrotherapy (to achieve an equivalent success rate in all patients), a predetermined model response is implicitly used, namely, the response to be produced in all of the patients that yields the desired probability of success. A patient-dependent electrical parameter, such as transthoracic impedance, that distinguishes the different patients, is used in preparing the translation factor that translates a nominal electrotherapy to an actual electrotherapy for a present patient.
One of the benefits of the present invention is that it avoids attempting to deliver inappropriately low energy levels to a patient with high impedance, and likewise, avoids delivering high-energy shocks to patients with low impedance which could result in excessive current flow, myocardial damage, and failure to defibrillate. Instead, a defibrillator constructed according to the present invention delivers defibrillation therapy tailored for the patient to have an equivalent probability of success for the patient as for other patients.